July, 1937
VAPOR PRESSURES OF AMMONIUM CHLORIDE SOLUTIONS
Summary 1. Silver chloride precipitated a t room temperature and aged in the mother liquor is subject to a rapid decrease of the surface as measured by the adsorption of wool violet. The aging is most pronounced in 0.001 M chloride in water as well as in ethanol, less as an equivalent body and the least in 0.001 M silver solution. 2. An explanation has been offered of the fact that silver chloride ages more rapidly as a "chloride body" than as an "equivalent body." Evidence has been presented that the Ostwald ripening is of subordinate significance.
[CONTRIBUTION FROM
THE
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3. Ammonia premotes the aging, the effect increasing with the concentration of ammonia. 4. Freshly precipitated silver chloride adsorbs about twice the amount of wool violet from a solution containing an excess of silver as from a solution containing an excess of chloride on the saturated surface. After one hour of aging the amount of dye adsorbed on the saturated surface is independent of the kind of lattice ion present in excess in the solution. An interpretation of this behavior has been presented. MINNEAPOLIS, MINN.
DIVISIONOF PHYSICAL CHEMISTRY OF
THE
RECEIVED MARCH11, 1937
STATE UNIVERSITY OF IOWA J
The Vapor Pressures and Activity Coefficients of Aqueous Solutions of Ammonium Chloride at 25'' BY J. N. PEARCE~ AND G. G. PUMPLIN Continuing the investigation of activity coefficients of salts in aqueous solution by the dynamic vapor pressure method devised by Pearce ~ * ~results and Snow, as previously d i ~ c u s s e d , the on ammonium chloride solutions are here reported. The attempt to apply the method to cupric sulfate solutions gave anomalous results. The apparatus used was exactly the same as reported by Pearce and B l a ~ k m a n . ~ Reagent grade ammonium chloride, purified by three recrystallizations, was made up into a nearly saturated solution, which was carefully analyzed gravimetrically for its chloride content by precipitation as silver chloride. Solutions of the required molality were made up on the weightmolal basis by dilution of the concentrated solution with conductivity water. The essential data are collected in Table I. The values of p given are the mean of three or more independent determinations, with a maximum deviation of 0.004 mm. from the mean. The symbols used have their familiar significance. The activity of the water, al, is calculated directly from the fact that al = p/pa. The geometric mean activity coefficients of the (1) Part of the dissertation presented to the Graduate College of the State University of Iowa by Gerald G. Pumplin in partial fulfilment of the requirements for the degree of Doctor of Philosophy. (2) Deceased Nov. 14, 1936. (3) Pearce and Nelson, THISJOURNAL, 64, 3544 (1932) : 66, 3075 (1933) (4) I'enrce and Blackman, rbid, 67, 24 (1935).
ions, y*, were calculated from the activity of the solvent by means of the relations
+
where h = (55.51 In ul/vm) 1, and vm is the total number of ions furnished by rn moles of solute. Randall6 has shown that for uni-univalent salts, a plot of h/ml/' against the square root of the molality has the limitingvalue of 0.394 at m = 0 for 25'. This plot is shown in Fig. 1. To calculate the value of the activity coefficient of any solution of molality m, the value of the integral from m = 0 to m was determined, using a polar planimeter, 0.4
0.3
-r 30.2 1\2
0.1
0.0 0.0
0.5
1.0
1.5
2.0
2.5 3.0
M1/*. Fig. 1.
The change in free energy accompanying the transfer of one mole of ammonium chloride from (5) Randall and White, ibid., 48, 2514 (1926). (6) Randall, ibid., 46, 2512 (1926).
J. N. PEARCE AND G. G. PUMPLIN
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any concentration m to a solution in which its activity is unity was calculated by the relation A% = RT In a2, where a2 = (y* m)2. The values of AZ,', the free energy change accompanying the dilution of a solution of concentration m to one exactly 0.1 m, are obtained by subtracting the value of AFz for m = 0.1 from the other values of A R . The change in free energy accompanying the transfer of one mole of solvent from the pure solvent to any solution in which its activity is a1 is calculated by means of the relation: AFl = RT In al. The activity of the solvent, the activity coefficients, and the free energy data are given in Table I. The variation of the activity coefficients with concentration is shown in Fig. 2, along with those for sodium chloride and 2.0 1.5
TABLE I VAPOR PRESSURE, ACTIVITY, AND FREEENERGY DATA OF AQUEOUSSOLUTIONS OF AMMONIUM CHLORIDEAT 25' m
P, mm.
a1
h/m'/'
-AK, Y t
cal.
-A 2 . l cal.
23.752 1.0000 0.394 1.000 ... , . . .2308 0.765 23.673 0.9967 1.98 0 .9934 3.93 752 .1787 ,721 23.595 23.438 ,9868 ,1209 .686 7.89 1515 .6 23.281 ,9802 ,0942 ,668 11.88 1964 .8 23.125 ,9736 ,0801 ,655 15.86 2282 22.970 ,9671 ,0708 .646 19.85 2530 1.0 22.588 .9510 ,0573 .628 29.79 2977 1.5 2.0 22.215 ,9353 ,0518 ,617 39.66 3298 2.5 21.841 ,9195 ,0433 ,610 49.73 3549 3.0 21.462 ,9036 .0357 ,605 60.11 3755 3.5 ,0294 ,604 70.63 3935 21.084 ,8877 4.0 ,8719 .0243 ,603 81.28 4092 20.709 4.5 20.338 .8563 ,0203 ,604 91.98 4234 .0172 19.974 .8409 5.0 ,604 102.69 4360 ,8105 ,0114 ,607 124.58 4581 19.250 6.0 7.3800" 18.281 ,7697 0057 .611 155.19 4834 Saturated.
0.0 .1 .2 .4
The determination of the activity coefficients
il.0
of cupric sulfate in solution in the same manner as
i--
0.5 I
0.0 0.0
VOl. 59
0.5
1.0
1.5
2.0
2.5 3.0
df. Fig. 2.-The change of activity coefficient with concentration: 1, sodium chloride; 2, ammonium choride; 3, potassium chloride.
potassium chloride. A minimum is noted in each case, the position of which indicates the relative degree of hydration of the cation. The lower the concentration at which the minimum occurs, the higher the degree of hydration. The order Na+ > K+ > NH4+ for the relative degree of hydration of these chlorides agrees with that obtained cryoscopically by Bourion and Rouyer,' and by Sugden.s (7) Bourion and Rouyer, Compl. rend., 197, 52 (1933). (8) Sugden, J . Chcm. SOC.,129, 174 (1926).
for ammonium chloride gave results which were not in good agreement with the best e. m. f. data Hydrolytic effects in the cupric sulfate solutions apparently render this method inapplicable.
Summary The vapor pressure over aqueous solutions of ammonium chloride is lowered from 23.673 mm. a t 0.1 m to 18.281 mm. at 7.3800 m (saturation). The activities of the solvent, the geometric mean activity coefficients of the solute, and the free energies of dilution have been calculated from the vapor pressure data. The method employed did not give satisfactory results using cupric sulfate. IOWA CITY,IOWA
RECEIVED MARCH6, 1937
(9) Nielson and Brown, THISJOURNAL, 49, 2423 (1927). (10) Getman, J Phys. Chcm.. 34, 1454 (1930). (11) Wetmore and Gordon, J . Chcm. Phys , 6, 60 (1937)
